Refactor black into packages in src/ dir (#1376)

- Move black.py to src/black/__init__.py
- Have setuptools_scm make src/_black_version.py and exclude from git
- Move blackd.py to src/blackd/__init__.py
- Move blib2to3/ to src/
- Update `setup.py`
- Update unittests to pass
  - Mostly path fixing + resolving
- Update CI
  - pre-commit config
  - appveyor + travis

Tested on my mac with python 3.7.5 via:
```
python3 -m venv /tmp/tb3
/tmp/tb3/bin/pip install --upgrade setuptools pip coverage pre-commit
/tmp/tb2/bin/pip install ~/repos/black/
cd ~/repos/black/
/tmp/tb2/bin/coverage run tests/test_black.py
/tmp/tb3/bin/pre-commit run -a
/tmp/tb3/bin/black --help
/tmp/tb3/bin/black ~/repos/ptr/ptr.py
```